Gas chromatography is essentially a physical method of separation in which constituents of a vapor sample in a carrier gas are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various components of the sample—which differ based upon differences among partition coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Temperature programming of the chromatographic column is a technique that has become common in some chromatographic analyses. Temperature programming can extend the range of analytes that can be separated during a single analysis, shorten analysis time, improve peak shape, and eliminate and/or reduce unwanted sample residue from the column after the chromatography has finished. At the end of the temperature program, the column oven is cooled back to the program's initial temperature so that it is ready for the next analysis. This cool-down step is a part of the analytical cycle, but it can represent a significant and unproductive portion of the total time needed to perform a sequence of analyses.
It has become fairly common in modern instrumentation to accelerate this cool-down process, which is performed by a chromatographic oven housing the column, so that the gas chromatograph can spend a greater proportion of its time on productive chromatography, thereby increasing the throughput of samples. Such acceleration of the cool-down process can result in significant benefits in terms of time and cost.
Accordingly, some teachings include oven designs that accelerate the cooling rate—in some cases, by a factor in the range of five to ten. However, in some instances, the carrier gas inside the column contracts during this rapid cooling at a rate faster than that at which the carrier gas is entering into the column. It has been discovered that this can produce a partial vacuum at the column outlet. Because the column outlet typically resides inside a detector, this vacuum will draw gases that are inside the detector back into the column during such rapid cooling, and these gases may be hostile to the column. For instance, in the case of a detector where combustion occurs, such as, for example, a flame ionization detector, a flame photometric detector, or a nitrogen-phosphorus detector, undesirable gases such as oxygen and water vapor may be drawn back into the outlet end of the column.
Several methods are available for preventing this ingress of the detector gases into the column during cooling. One such method entails introducing a small flow of a “make-up” gas between the column and the detector. This would ensure that, as the gas in the column contracts, only pure carrier gas would enter the column exit. However, this approach requires the use of an additional carrier gas supply, which is undesirable due to the concomitant extra cost and complexity necessary to install and operate an extra gas supply.
Additionally, some columns generate significant stationary phase bleed when operated at temperatures close to their specified limit. A fast cooling oven can “chill” this bleed, causing it to collect in pockets along the column. The next time the column is temperature programmed, these focused areas of bleed may manifest themselves as ‘ghost peaks’ on the chromatogram.